A number of clipless pedal designs have been made commercially available. The first clipless pedals were single side entry because they had latch and release mechanisms on only one side of the pedal. The next clipless pedals were double side entry because they had latch and release mechanisms on opposite sides of the pedal. The advantage of double side entry is that it is easier for the rider to clip into the pedal without looking. The disadvantage is that double side entry pedals are generally more complex and heavier than single side entry pedals.
A cleat is attached to the bottom of each cycling shoe and when the rider steps onto the clipless pedal and applies force, the latch mechanism opens to accept the cleat and then latches shut to lock the cleat to the pedal. The rider releases himself or herself from the pedal by twisting their shoe past a given angle. Pedal/cleat designs can allow float or not depending on the pedal and cleat design. Float is when the cleat is free to pivot through some angle such as 5 degrees each direction before making contact with the release mechanism. Some riders prefer to have float because of knee comfort and others prefer no float. Some pedals offer one cleat that allows float and a different cleat that allows no float.
Clipless pedals either have the latch mechanism in the pedal or in the cleat which is attached to the shoe. Designs with the latch mechanism in the cleat are generally lighter but perform poorly in muddy conditions because cleats are usually larger and have moving parts and are prone to getting clogged with mud and dirt from occasional walking on the ground. Also, because they are usually larger, they can interfere with walking. Designs with the latch mechanism in the pedal generally consist of a relatively large number of components making them complex to manufacture and maintain, and relatively bulky and heavy. Also, while designs with latch mechanisms in the pedal are less prone to clogging with mud than designs with the latch mechanism in the cleat, most are still susceptible to clogging under certain conditions which makes some pedals difficult to lock into and others difficult to clip out of.
In order to prevent release while pulling upwards while pedaling, either the spring(s) that holds the mechanism closed needs to be very strong or the geometry must be designed such that when the cleat is pulled straight up, there is not a component of force to cause the mechanism to release.
Spring loaded latch mechanisms on existing pedal designs have clasps that pivot about axes that are not coincident with the axis of pedal rotation. There is some distance between the pedal axis and the clasp axis or axes. Because of this, existing pedal designs have a framework to support the clasp axle and this causes the pedal to be relatively bulky in size.